It is well known in the art to compensate for hearing losses mainly caused by deficiencies in a person's outer or middle ear by converting received sounds to vibrations and transmitting the vibrations to the person's head. The bone structure of the skull leads the vibrations to the person's inner ear and thus enables the person to perceive the sounds. It is also known to use the same principle for compensating for single-sided deafness by placing the microphone receiving the sounds close to the person's deaf ear and letting the skull bone lead the vibrations to the opposite, intact inner ear.
A well-known type of bone-conduction hearing devices comprises a vibrator, which is pressed against the skin of the person's head by means of a spring or an elastic headband, and which transmits the vibrations to the skull bone through the skin and the subcutaneous tissue (transcutaneous transmission). Another well-known type of bone-conduction hearing devices comprises a vibrator detachably coupled to a fixture implanted (osseointegrated) in the skull bone. The vibrator transmits the vibrations to the skull bone through the fixture (percutaneous transmission).
For both types of bone-conduction devices, a precise determination of the magnitude of the vibrations applied to the skull bone is needed for determining a person's bone-conduction hearing thresholds as well as for calibrating the hearing devices. Therefore, various attempts have been made to develop devices and methods for determining the vibrational force and/or the vibrational acceleration.
The dissertation, “Contributions to a better understanding of fitting procedures for Baha”, Hodgetts, William E., Ph.D., UNIVERSITY OF ALBERTA, 2008, NR45445, discloses a device for measuring a vibrational acceleration. The device comprises a vibrator (“BEST” transducer) with a stiff vibration element placed within a housing also acting as countermass. The vibration element comprises a coupling for the implanted fixture on one side of the housing and protrudes on the opposite side of the housing, where an accelerometer is attached to the vibration element. The accelerometer thus vibrates together with the vibration element, and its output signal represents the acceleration of the vibration element. Since, however, the mechanical impedance, or admittance, of the coupling is not well known and further may change, e.g. due to aging of the used materials and/or the person's tissue and bone structure, the correlation between the output of the accelerometer and the vibrational force applied to the skull lacks the desired precision.
It is an object of the present invention to provide a device and a method for applying a vibration signal to a human skull bone, which device and method allow determination of the applied vibrational force with better precision than prior art devices and methods.